The direct conversion radio architecture is a popular choice for radio frequency (RF) transceiver design. It uses fewer components and is easier to integrate compared to the conventional heterodyne architecture. See, e.g., Mirabbasi et al., “Classical and modern receiver architectures,” IEEE Commun. Mag., vol. 38, no. 11, pp. 132-139, 2000. Those benefits also come with several well-known impairments, namely in-phase/quadrature (I/Q) imbalance and direct current (DC) offset.
In a direct conversion transceiver, a quadrature modulator (QM) implements I/Q modulation and RF upconversion, while a quadrature demodulator (QDM) realizes I/Q demodulation and RF downconversion. Because of the limitation in analog circuit precision, the quadrature carriers used in QM and QDM do not have exactly the same amplitude and a perfect 90° phase difference. Similarly, the analog reconstruction filters on the I and Q paths may not match exactly. These imperfections are called I/Q imbalance, which causes cross talk between I and Q channels and which creates undesired images of the original signal. Addition, because of limited isolation in analog components, some of the local oscillator (LO) power can leak into RF output, which creates LO spikes in transmitted signal and DC offset in received signal.
Most existing compensation techniques treat transmit and receive I/Q imbalance separately. To compensate for I/Q imbalance in the transmitter, the receive side is either assumed to have dedicated feedback loops or digital demodulators with perfect quadrature carriers. See, e.g., Cavers et al., “Adaptive compensation for imbalance and offset losses in direct conversion transceivers,” IEEE Trans. Veh. Technol., vol. 42, no. 4, pp. 581-588, 1993; Faulkner et al., “Automatic adjustment of quadrature modulators,” Electron. Lett., vol. 27, no. 3, pp. 214-216, 1991; Ding et al., “Compensation of Frequency-Dependent Gain/Phase imbalance in predistortion linearization systems,” IEEE Trans. Circuits Syst., vol. 55, no. 1, pp. 390-397, 2008; and Anttila et al., “Frequency-Selective I/Q mismatch calibration of wideband Direct-Conversion transmitters,” IEEE Trans. Circuits Syst., vol. 55, no. 4, pp. 359-363, 2008. To compensate for I/Q imbalance in the receiver, the transmit side is usually assumed to be free of I/Q imbalance. See, e.g., Anttila et al., “Circularity-Based I/Q imbalance compensation in wideband Direct-Conversion receivers,” IEEE Trans. Veh. Technol., vol. 57, no. 4, pp. 2099-2113, 2008. Each of these different approaches, however, has undesirable effects and drawbacks.
Some other conventional techniques are as follows: U.S. Pat. Nos. 7,463,866; 7,567,611; and 7,567,783.